In a refrigeration system including a compressor, a condenser, a fixed throttle valve, and an evaporator having an outlet port connected to an accumulator, the proper volume of refrigerant in the refrigeration system must be maintained. If the proper volume of refrigerant is not enclosed in the refrigeration system problems occur. If too much refrigerant is enclosed within the system, refrigerant in the liquid phase can be forced from the accumulator into the compressor because the capacity of the accumulator is too small, and the compressor can suffer damage by compressing liquid refrigerant. Also, the compression capacity may decrease. On the other hand, if too little refrigerant is in the refrigeration system, there may be no refrigerant in the accumulator. The temperature of the refrigerant at an inlet port of the compressor therefore may increase, and the refrigeration capacity of the evaporator will decrease.
If the accumulator is used in an automotive air conditioner, the volume of the accumulator depends on the size of the engine compartment and on operational factors which cause the amount of refrigerant in the accumulator to vary. Thus, the accumulator should be small enough to fit in the engine compartment, but be sized to accommodate refrigerant throughout the range of rotational speeds of the compressor, throughout the range of thermal loads on the evaporator, and throughout the range of thermal loads on the condensor. If properly sized, the accumulator will be filled with refrigerant when the rotational speed of the compressor is the lowest (i.e, at engine idle), when the thermal load on the evaporator is lowest (e.g., when air entering the evaporator has a temperature of 20.degree. C. to 25.degree. C. and relative humidity of about 50%), and when the thermal load on the condenser is highest (e.g., air temperature at the condenser inlet is 30.degree. C. to 35.degree. C.). When the rotational speed of the compressor and the thermal load on the evaporator are the highest (e.g., air temperature at the inlet is 30.degree. C. to 35.degree. C. and relative humidity is about 50%), there will be virtually no refrigerant in the accumulator.
In a conventional method for measuring the volume of refrigerant, the surface temperature of the outlet port of the evaporator is initially measured to determine the proper refrigerant amount (if the temperature is sufficiently low, the refrigerant volume is proper). However, this method has a very low degree of precision.
Another method for detecting low refrigerant levels involves measuring the clutch cycling time (the duration the compressor clutch is on and off). The clutch cycling time is shortened when the refrigerant volume is low. However, since the clutch cycling time can be shortened by other factors as well, it is an imprecise method for detecting the lack of refrigerant.
One apparatus for detecting the proper amount of refrigerant is disclosed in Japanese Utility Model Application Publication No. 57-33343. In this application, as illustrated in FIG. 1, accumulator 6 comprises housing 61 having an open end covered by end plate 62. Inlet port 7 is disposed on the upper portion of housing 61 and communicates between the inside of housing 61 and an evaporator (not shown). Outlet tube 8 communicates between the inside of housing 61 and a compressor (not shown), and a conduit 63 vertically extends inside housing 61. End plate 62 has depression 64 which is covered by sight glass 65 to form a small space. One end of conduit 63 is open to depression 64 which communicates with outlet tube 8 through passageway 66. The other end of conduit 63 extends to the lower portion of housing 61 to draw gaseous refrigerant and entrained lubricating oil to outlet tube 8 though depression 64 and passageway 66. Accordingly, if the liquid refrigerant passes through depression 64 its existence can be confirmed through sight glass 65. However, liquid refrigerant may flow past sight glass 65 even when refrigerant volume is improper. Moreover, because conduit 63 and passageway 66 return some lubricating oil to housing 61, liquid can be detected whether or not refrigerant exists in the lower portion of housing 61. In addition, when the volume of accumulator 6 is minimized as required, it is difficult to properly detect refrigerant therein by this method.
Another known accumulator 6, as shown in FIG. 2, includes inlet port 7 and U-shaped tube 9 integrally formed with outlet port 8. U-shaped tube 9 is provided with lower opening 91 for returning lubricating oil to the accumulator and upper opening 92 for equalizing pressure. Immediately after the compressor stops, a pressure difference is produced between the compressor and accumulator 6 causing liquid refrigerant to flow into U-shaped tube 9 through hole 91 and into the inside of the compressor. Since U-shaped tube 9 is provided with hole 92 to equalize the pressure difference, the compressor does not does not compress liquid refrigerant when the compressor is restarted. However, in the above accumulator, it is impossible to detect the refrigerant volume in the refrigeration system.